There are a number of types of apparatus where a plurality of chambers or systems need to be evacuated down to different levels of vacuum. For example, in well known types of mass spectrometer, the analyser/detector has to be operated at a relatively high vacuum, for example 10−5 mbar, whereas a transfer chamber, through which ions drawn and guided from an ion source are conveyed towards the detector, is operated at a lower vacuum, for example 10−3 mbar. The mass spectrometer may comprise one or more further chambers upstream from the analyser chamber, which are operated at progressively higher pressures to enable ions generated in an atmospheric source to be captured and eventually guided towards the detector.
Whilst these chambers may be evacuated using separate turbo-molecular vacuum pumps, each backed by a separate, or common backing pump, for example a rotary vane pump, it is becoming increasingly common to evacuate two or more adjacent chambers using a single, “split flow” turbo-molecular pump having a plurality of inlets each for receiving fluid from respective chamber, and a plurality of pumping stages for differentially evacuating the chambers. Utilising such a pump offers advantages in size, cost, and component rationalisation.
For example, EP-A 0 919 726 describes a split flow pump comprising a plurality of vacuum stages and having a first pump inlet through which gas can pass through all the pump stages and a second inlet through which gas can enter the pump at an inter-stage location and pass only through a subsequent stage of the pump. The pump stages prior to the inter-stage location are sized differently from those stages subsequent to the inter-stage location to meet the pressure requirements of the different chambers attached to the first and the second inlets respectively.
However, when mounted to a mass spectrometer in a conventional manner, for example with the axis of the pump, or more particularly, its shaft axis, either parallel to or perpendicular to the plane of the outlet flanges of the mass spectrometer, conductance limitations of such a split flow pump compromise performance in comparison to an arrangement where adjacent chambers are evacuated using a bespoke vacuum pump directly mounted on to the respective chamber.
For example, when the pump is orientated with respect to the mass spectrometer such that the shaft axis is parallel to the plane of the outlet flanges, then gas must flow around a right angle bend to enter the pump inlet, which results in a pressure drop and associated loss of pumping speed. When the vacuum pump is orientated with its shaft axis perpendicular to the plane of the inlet of the outlet flange, whilst gas may flow easily into the first inlet, gas must flow around two bends in order to enter the second pump inlet.
In EP-A 1 085 214, these problems are reduced by mounting the split flow pump to the bottom of the mass spectrometer such that the shaft axis is inclined at an angle to the plane of the outlet flanges. With this orientation, the gas flows into the inlets by flowing around bends of obtuse angle so that there is little pressure drop between the outlet flanges and the pumping inlets. However, with such an arrangement the overall volume occupied by the mass spectrometer and split flow pump is increased in comparison to an arrangement where the shaft axis is parallel to the outlet flanges.
It is an aim of at least the preferred embodiment of the present invention to seek to provide an improved arrangement for the differential evacuation of a multi-chambered system, such as a mass spectrometer.